A light-emitting element is high in visibility and optimum for low profiling since it emits light and does not require any backlight which is required in a liquid crystal display device (LCD), and that has no limitation in visual angle. Therefore, in recent years, a light-emitting device using the light-emitting element has attracted attention as a display device alternative to a CRT and the LCD. In addition, as used herein, the light-emitting element means an element whose luminosity is controlled by a current or a voltage, and an OLED (Organic Light Emitting Diode), an MIM type electron source element (electron emitting element) used in and an FED (Field Emission Display), and the like are fall within the definition.
The light-emitting device includes a panel and a module having an IC or the like including a controller mounted on the panel. This invention also relates to an element substrate equivalent to one mode achieved before a completion of the panel in a process of manufacturing the light-emitting device, and each of pixels in the element substrate is provided with a means for supplying a current to the light-emitting element.
The OLED (Organic Light Emitting Diode) which is a variation of the light-emitting element has a layer comprising an electro-luminescent material capable of obtaining luminescence (electro-luminescence) generated upon application of an electric field (hereinafter referred to as an electro-luminescent layer), an anode layer, and a cathode layer. The electro-luminescent layer is provided between the anode and the cathode and constituted of a layer or a plurality of layers. In some cases, an inorganic compound is contained in the layer or layers. A light emission (fluorescence) generated when a singlet excitation state returns to a ground state and a light emission (phosphorescence) generated when a triplet excitation state returns to a ground state are included in the luminescence in the electro-luminescent layer.
Hereinafter, a structure of a pixel of an ordinary light-emitting device and driving of the pixel will be described briefly. The pixel shown in FIG. 7 has a switching transistor 700, a driving transistor 701, a capacitance element 702, and a light-emitting element 703. A gate of the switching transistor 700 is connected to a scan line 705, and a source thereof is connected to a signal line 704 when a drain thereof is connected to a gate of the driving transistor 701. A source of the driving transistor 701 is connected to a power line 706, and a drain thereof is connected to an anode of the light-emitting element 703. A cathode of the light-emitting element 703 is connected to a counter electrode 707. The capacitance element 702 is provided in such a manner as to retain a potential difference between the gate and the source of the driving transistor 701. Predetermined voltages are applied separately to the power line 706 and the counter electrode 707, so that the power line 706 and the counter electrode 707 have a potential difference therebetween.
When the switching transistor 700 is turned on by a signal from the scan line 705, a video signal input to the signal line 704 is input to the gate of the driving transistor 701. A potential difference between a potential of the input video signal and the power line 706 becomes a gate/source voltage Vgs, so that a current is supplied to the light-emitting element 703 to cause the light-emitting element 703 to emit light.